Field-Induced Staggered Magnetic Order in La(2)NiO(4.133)

At low temperature the holes doped into the NiO(2) planes of La(2)NiO(4.133) by the excess oxygen collect in diagonal stripes that separate narrow antiferromagnetic domains. The magnetic order drops abruptly to zero at T_m = 110.5 K, but charge order remains with a period of 3a/2. We show that application of a magnetic field in the regime T > T_m induces staggered magnetic order of period 3a due to the net magnetic moment of the high-temperature bond-centered stripes, together with the odd number of Ni spins across an antiferromagnetic domain.Comment: 4 pages, Revtex, 4 epsf figs included with psfig

Schwinger-Boson Mean-Field Theory of Mixed-Spin Antiferromagnet L2BaNiO5L_2BaNiO_5

The Schwinger-boson mean-field theory is used to study the three-dimensional antiferromagnetic ordering and excitations in compounds $L_2BaNiO_5$, a large family of quasi-one-dimensional mixed-spin antiferromagnet. To investigate magnetic properties of these compounds, we introduce a three-dimensional mixed-spin antiferromagnetic Heisenberg model based on experimental results for the crystal structure of $L_2BaNiO_5$. This model can explain the experimental discovery of coexistence of Haldane gap and antiferromagnetic long-range order below N\'{e}el temperature. Properties such as the low-lying excitations, magnetizations of $Ni$ and rare-earth ions, N\'{e}el temperatures of different compounds, and the behavior of Haldane gap below the N\'{e}el temperature are investigated within this model, and the results are in good agreement with neutron scattering experiments.Comment: 12 pages, 6 figure

Energy Storage in Electrochemical Cells with Molten Sb Electrodes

An energy-storage concept is proposed using molten Sb as the fuel in a reversible solid-oxide electrochemical cell (SOEC). Because both Sb and Sb2O3 are liquids at typical SOEC operating temperatures, it is possible to flow Sb from an external tank and use it as the fuel under fuel-cell conditions and then electrolyze Sb2O3 during recharging. This concept was tested using a button cell with a Sc-stabilized zirconia electrolyte at 973 K by measuring the impedances under fuel-cell and electrolyzer conditions for a range of stirred Sb-Sb2O3 compositions. The Sb-Sb2O3 electrode impedances were found to be on the order of 0.15 Ωcm2 for both fuel-cell and electrolyzer conditions, for compositions up to 30% Sb and 70% Sb2O3. The open circuit voltages (OCV) were 0.75 V, independent of oxygen composition. Some features of using molten Sb as an energy-storage medium are discussed

Effect of the Orbital Level Difference in Doped Spin-1 Chains

Doping of a two-orbital chain with mobile S=1/2 Fermions and strong Hund's rule couplings stabilizing the S=1 spins strongly depends on the presence of a level difference among these orbitals. By DMRG methods we find a finite spin gap upon doping and dominant pairing correlations without level-difference, whereas the presence of a level difference leads to dominant charge density wave (CDW) correlations with gapless spin-excitations. The string correlation function also shows qualitative differences between the two models.Comment: 4 pages, 4 figure

Magnetic transitions in Pr2NiO4 single crystal

The magnetic properties of a stoichiometric Pr2NiO4 single crystal have been examined by means of the temperature dependence of the complex ac susceptibility and the isothermal magnetization in fields up to 200 kOe at T=4.2 K. Three separate phases have been identified and their anisotropic character has been analyzed. A collinear antiferromagnetic phase appears first between TN = 325 K and Tc1 = 115 K, where the Pr ions are polarized by an internal magnetic field. At Tc1 a first modification of the magnetic structure occurs in parallel with a structural phase transition (Bmab to P42/ncm). This magnetic transition has a first‐order character and involves both the out‐of‐plane and the in‐plane spin components (magnetic modes gx and gxcyfz, respectively). A second magnetic transition having also a first‐order character is also clearly identified at Tc2 = 90 K which corresponds to a spin reorientation process (gxcyfz to cxgyaz magnetic modes). It should be noted as well that the out‐of‐phase component of χac shows a peak around 30 K which reflects the coexistence of both magnetic configurations in a wide temperature interval. Finally, two field‐induced transitions have been observed at 4.2 K when the field is directed along the c axis. We propose that the high‐field anomaly arises from a metamagnetic transition of the weak ferromagnetic component, similarly to La2CuO4

Mid-Infrared Conductivity from Mid-Gap States Associated with Charge Stripes

The optical conductivity of La(2-x)Sr(x)NiO(4) has been interpreted in various ways, but so far the proposed interpretations have neglected the fact that the holes doped into the NiO(2) planes order in diagonal stripes, as established by neutron and X-ray scattering. Here we present a study of optical conductivity in La(2)NiO(4+d) with d=2/15, a material in which the charge stripes order three-dimensionally. We show that the conductivity can be decomposed into two components, a mid-infrared peak that we attribute to transitions from the filled valence band into empty mid-gap states associated with the stripes, and a Drude peak that appears at higher temperatures as carriers are thermally excited into the mid-gap states. The shift of the mid-IR peak to lower energy with increasing temperature is explained in terms of the Franck-Condon effect. The relevance of these results to understanding the optical conductivity in the cuprates is discussed.Comment: final version of paper (minor changes from previous version

Random interactions and spin-glass thermodynamic transition in the hole-doped Haldane system Y2−x_{2-x}Cax_xBaNiO5_5

Magnetization, DC and AC bulk susceptibility of the $S$=1 Haldane chain system doped with electronic holes, Y$_{2-x}$Ca$_x$BaNiO$_5$ (0$\leq$x$\leq$0.20), have been measured and analyzed. The most striking results are (i) a sub-Curie power law behavior of the linear susceptibility, $\chi (T)$$\sim$ $T$$^{-\alpha}$, for temperature lower than the Haldane gap of the undoped compound (x=0) (ii) the existence of a spin-glass thermodynamic transition at $T$$_g$ = 2-3 K. These findings are consistent with (i) random couplings within the chains between the spin degrees of freedom induced by hole doping, (ii) the existence of ferromagnetic bonds that induce magnetic frustration when interchain interactions come into play at low temperature.Comment: 4 pages, 4 figures, to appear in Phys. Rev.

Doped two orbital chains with strong Hund's rule couplings - ferromagnetism, spin gap, singlet and triplet pairings

Different models for doping of two-orbital chains with mobile $S=1/2$ fermions and strong, ferromagnetic (FM) Hund's rule couplings stabilizing the S=1 spins are investigated by density matrix renormalization group (DMRG) methods. The competition between antiferromagnetic (AF) and FM order leads to a rich phase diagram with a narrow FM region for weak AF couplings and strongly enhanced triplet pairing correlations. Without a level difference between the orbitals, the spin gap persists upon doping, whereas gapless spin excitations are generated by interactions among itinerant polarons in the presence of a level difference. In the charge sector we find dominant singlet pairing correlations without a level difference, whereas upon the inclusion of a Coulomb repulsion between the orbitals or with a level difference, charge density wave (CDW) correlations decay slowest. The string correlation functions remain finite upon doping for all models.Comment: 9pages, 9figure

Structure of end states for a Haldane Spin Chain

Inelastic neutron scattering was used to probe edge states in a quantum spin liquid. The experiment was performed on finite length antiferromagnetic spin-1 chains in Y_2BaNi_{1-x}Mg_xO_5. At finite fields, there is a Zeeman resonance below the Haldane gap. The wave vector dependence of its intensity provides direct evidence for staggered magnetization at chain ends, which decays exponentially towards the bulk (xi = 8(1) at T=0.1K). Continuum contributions to the chain end spectrum indicate inter-chain-segment interactions. We also observe a finite size blue shift of the Haldane gap.Comment: 4 pages RevTex, 3 figure

Coexistence of Haldane gap excitations and long range antiferromagnetic order in mixed-spin nickelates R_2 Ba Ni O_5

The spin dynamics of the S=1 Ni-chains in mixed-spin antiferromagnets Pr_2 Ba Ni O_5 and Nd_x Y_2-x Ba Ni O_5 is described in terms of a simple Ginzburg-Landau Lagrangian coupled to the sublattice of rare-earth ions. Within this framework we obtain a theoretical explanation for the experimentally observed coexistence of Haldane gap excitations and long-range magnetic order, as well as for the increase of the Haldane gap energy below the Neel point. We also predict that the degeneracy of the Haldane triplet is lifted in the magnetically ordered phase. The theoretical results are consistent with the available experimental data.Comment: 4 pages, 1 figure, submitted to PRL An alternative derivation of main results and new references adde